The use of lasers for spectroscopy, for document scanning and for determining direction and range for objects moving in space (i.e. LIDAR) is known in the prior art. In each of these systems, light, generated by a laser is detected by electronic apparatus to make a measurement.
It has been recognized for some time that excess noise (i.e. noise above the shot noise level), spurious modulation and power drift in the laser can significantly reduce the accuracy of many measurements of laser light. In gas lasers, the noise levels can easily reach 50 dB above shot noise even at relatively high frequencies.
Shot noise is a random current fluctuation that occurs when light is detected. Conventional photodetectors, such as PIN diodes, pass current from their anode to their cathode in proportion to the number of photons that strike the detector. Each photon can generate only one electron. The random current fluctuations occur because the arrival of photons at the photodetector may be accurately modeled as a Poisson process. For any given time interval, there is an average number of photons that are expected to arrive at the detector and a standard deviation representing nominal variations from the average. It is this variation in the arrival of photons, causing a corresponding variation in the current passed by the photodetector, which causes the shot noise.
Since, however, the level of noise signals is usually the highest at low modulation frequencies, many high-precision optical measurement systems apply some sort of modulation to the beam. This makes the output signals of the measurement system periodic in time at a frequency that is sufficiently far above the low-frequency noise signals to substantially reduce the effect of the noise on the measurement.
Another method of reducing noise is to take a sample of the output beam and apply negative feedback to the laser operating current or to an external optical attenuator to keep the photocurrent, derived from the sample beam, constant. These systems tend to be complicated or expensive and can, at best, bring the signal-to-noise ratio of the output beam up to the signal-to-shot-noise ratio of the sample beam. Since, in these systems, the sample beam is usually appreciably weaker than the output beam and, so contains relatively more shot-noise, this method may not provide acceptable levels of noise reduction. In addition, since these systems depend on feedback, the effective bandwidth of the noise-reduced beam is often relatively small.
All-electronic noise suppression schemes have been known for some time. These schemes differ from those described above in that no attempt is made to stabilize the laser beam itself, only the photocurrent of the detected laser light. In these systems, a laser beam is typically split into a signal beam and a sample beam. The signal beam is passed through the optical system to one detector while the sample beam is passed around the optical system to another detector.
After detection, signal components in the signal beam emerging from the optical system which are also in the sample beam are cancelled electronically by subtraction or division. In the ideal, a system of this type produces an output current which represents a noise-free measurement.
These electronic methods rely on two important properties of the optical system: wide temporal bandwidth and highly linear photodetectors. Due to the wide bandwidth, the optical system does not introduce any differential gain or differential phase to the modulation of the beam as long as the relative path delays between the signal and sample beams are small. In this arrangement, the instantaneous fractional excess amplitude noise of the sample beam is substantially the same as that of the signal beam. The linearity of the photodetectors ensures that this is true for the photocurrents as well; thus if the DC components of the two photocurrents cancel, the excess noise components of the photocurrents also cancel at all frequencies of interest.
Most conventional all electronic noise cancellation systems either subtract the detected sample photocurrent from the signal photocurrent or divide the signal photocurrent by the sample photocurrent to achieve the noise reduction.
In the subtractive schemes, the optical system is desirably adjusted so that the photocurrents from the detected signal and sample beams are exactly equal. The result of the subtraction operation is a current representing the variations in the signal beam, without the DC component or any of the excess noise signal components of the signal beam. Subtracter can be designed to have relatively wide bandwidths since the photocurrents may be subtracted directly without prior conversion to voltages.
The addition of a properly adjusted subtracter can reduce the excess noise in a system by approximately 20 dB. This seems to be an practical limit on the improvement that can be obtained, however, since conventional subtraction systems require finicky adjustment of either the beam sampler, the optical system or the gain of one of the detector channels. Furthermore, because the steady-state intensity of the signal beam usually varies somewhat during a measurement, any null that is achieved by this process may be degraded during the operation of the device. In addition, the shot noise currents of the detected signal and sample photocurrents are uncorrelated and both contribute noise to the output current. Thus, the noise floor of the output current is limited to about 3 dB above the level of the shot noise in the signal photocurrent alone.
Dividing noise cancellation systems at first may appear to be more desirable than subtractive systems since they do not need precise adjustment and since they cancel fractional rather than absolute noise deviations. Unfortunately, dividers tend to be relatively slow, limiting the bandwidth of the noise suppressed signal. In addition, since dividers are inherently noisy, dividing may add significant amounts of noise to the signal.
U.S. Pat. No. 4,896,222 to Fukai relates to a system in which the sample photocurrent is electrically corrected and then subtracted from the signal photocurrent.
U.S. Pat. No. 4,7l8,121 to Epworth concerns a noise reduction system which uses variable gain amplifiers to reduce laser noise by subtraction. This system operates on two beat-frequency signals, one of which is the sum of a signal beam and a local oscillator beam and the other is the difference between these two beams. The system measures the noise and adjusts the gains of the two amplifiers to cancel the noise when the signals are summed.
U.S. Pat. No. 4,150,402 to Tietze et al. relates to a system which uses the sample photocurrent as a control signal for a variable gain amplifier which amplifies the signal photocurrent.